Microphone



Get. 2, 1928., v

H. LHCHTE MICROPHONE:

Filed Au so, 1925 amas ?atcnted Get. 2, 1%28.

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HUGO LICHTE, 9F KIEL, GERMANY, ASSIGNOR TO SIGNAL GESELLSGHAZFT MIT nnscnnanntrnn narrows, or mun, GERMANY.

MICBGIPHONE.

Application filed August 30, 1923, Serial No.

This invention relates to apparatus and arrangements, such as microphones or the like, in which fluctuating currents are pro- 'duced in variations of the electrical conduca tivity o a substance or a mixture or composition of substances inserted between elcctrodes, the said variations of conductivity.

being caused by alternating mechanical impulses applied to the electrodes. Preferably 10 the invention contemplates arrangements of the kind which are built up as vibratory structures of a definite natural frequency,

such as tuned microphones.

A well known drawback in apparatus of this character is the chai'igcability of their natural frequency. It is the object of the invention to diminish or entirely to an nihilate this changeability. The invention is based upon the iollpwing deliberation:

The primary object of my invention is to provide in a vibratory structure for transforming mechanical vibrations into electric current variations, an improved relation be tween, and arrangement of the masses in difi'erent portions of said structure, one of said portions having a. constant elastic force and the other of said portions having a variable elastic force. More specifically stated,

this improved relation and arrangement contemplates such a preponderance in the size of the portion having a constant elastic force over the size of the portion having a variable elastic force that any variation in the variable elastic force in the latter will not produce in the vibratory structure as a whole a greater variation of elastic force than ispermissible under a predetermined limit.

A microphone constructed according to the principles of the present invention is cspeciall'y adapted for signalling purposes, my

invention in such an adaptation making t possible to maintain the natural tuning of the microphone constant as permanently as possible. The function of a microphone .45 constructed in this manner is to be found wherever it is desirable to employ a microphone having a constantly uniform natural tuning.

Mechanical oscillator; structures of this so kind comprise masses 1 and elasticity E upon the amount of which depends their natural frequency. Masses and elasticity are defined by the dimensions of the structural parts of the apparatus. Themasses are, as

860,114, and in Germany September 7, 1922.

the component E is a substantial part of the total elasticity E of the elastic system ,vehement variations of the natural frequency of the said system are caused by the variations of E The invention consists in designing the oscillatory structure represented by the apparatus so that the elasticity of the variable substance, for instance of the granulated carbon within the microphone, is small in comparison with the total elasticity of the system, i. e; I I

The meaning of this deliberation may be elucidated by the example of an oscillatory microphone of definite natural frequency for a definite maximum current. Cross section and height of the carbon chamber are given by the maximum current. sults a definite quantity 9/ of granulated carbon the elasticity E of which can be found by calculation or by experiment. Besides may be given the desired natural frequency of the microphone and the allowable differ-once r of this natural frequency in percents of this frequency.

By experience the usual variations of the elasticity of the carbon contents are known which all may easily be found by experiments under SlllllifiT'COl'ldlllOIlS as they arise for microphones in normal use.

For the natural frequency of microphone the following equation exists:

in which it is a cousant, E the total elasticity, and M the total mass of the oscillatory structure.

E in this equation may be divided into two components E and E, E, being the in- There'lrom re- 8 Then The allowable difference in the natural' frequency may be irn. The usual variation of the elasticity of the carbon contents ma be found as 17715,.

quation (2) thus becomes In n Ei LZiEfl from which in connection with Equation (1) the values- "and M may be calculated, which must e xist as a minimum for'the given frequencyva'. The approximate values E for M and are p g and Therefrom the en 'neer may find out the dimensions'of the diaphragm for the microphone and of the mass of the oscillating structural elements, if p, r, n and E are given. I

In the drawing an example of the invention is shown represented by an oscillatory microphone connected to an oscillating diaphragm.

A casing with an aperture in the front is closed by an oscillatory diaphragm 1 upon which the microphone is mounted. The microphone consists of two masses 3 and t and a diaphragm 2, themass 3 being shaped as a casing enclosing the chamber for the granulated carbon contents 5.

For microphones which are usedin connection with other oscillato structures (diaphragm 1 or the like) an which consist of twomasses (3, 4) connected by an elastic member (diaphragm 2) it is usually sufiicient to calculate the amount of the so called free mass 4 instead of the total mass. This rule'exists especially for microphones of submarine sound receivers where the mass 3 is relatively large compared with the freel oscillating mass 4 because ,3 is compose of a relatively large structural mass mounted upon the receiving dia hragm 1, of a relatively large part of this iaphragm itself and of a remarkable volume of water co-oscillating with diaphragm 1.

Suflicient results with regard. to the conif E. is about one fifth of the total elasticity E of the oscillatory structure represented by the microphone.

For the usual currents of 100 milliamperes, i. e. for the usual volume of granulated carbon of such microphones and for the usual frequency of submarine signaling of about 1000 oscillations er second is given afreely vibrating mass of about four grams or more for a loosejcoupling ofthe microphone with the receiving diaphragm (a coupling of about 10%).

The invention may be applied to microphones and detectors of any kind in which the variable resistance represents at the same time a variable part of the" elasticity of the oscillating system.

. I claim: I

1. A microphone comprising a substantially rigid casing, an elastic diaphragm haying a constantelastic force forming one side of said casing, an electrode secured to said casing, a second electrode secured to said diaphragm in proximity and parallel to said first electrode, a filling of granular car:

vbon substance having a variable elasticity included between said electrodes, the elastic force of the said diaphragm being substan-' tially four times the elastic force of said granular filling. v

. 2. An ap aratus for transforming mechanical vibrations into electric current variations comprising a vibratory structure composed of a rigid member, an. elastic member having a constant elastic force, supported by said rigid member, a air of electrodes, one of said electrodes being mounted on said rigid member, the second electrode, mounted on said elastic member granular carbon substance having a vari-a le elastic force included between said air of elec-' trodes, the elastic force of sai carbon substance being small compared to the elastic force of said elasticmember.

3. A submarine sound'receiver comprising a watertight casing, a diaphragm, formin part of said casing, a micro hone connecte to said diaphragm and wit in) said casing comprising a rigid member attached. to said diaphragm, an elastic member having 'a. com 116 stant elastic force su ported by said rigid member, two electro es mounted between and respectively on said ri 'd and said elastic members, a granular car on substance having a variable elastic force included be- 120 tween said electrodes, theelastic force of the saidcarbon substance bein less than onefourth of the total elastic force of the receiver.

4. A microphone of the type described in- 126 eluding a vibrating structure possessing a substantially constant elastic force 'and microphonic material operating in conjunction with said vibrating structure upon the receiptof a signal possessing a varying elastic force, the said-constant elastic force being substantially the greater part of the total elastic force of the said microphone,

whereby the frequency of said microphone is at all times substantially unchanged.

5. A microphone of the type described, including a vibrat ng structure possessing a substantially constant elastic .force and microphonic material operating in conjunction with said vibrating structure upon the receipt of a signal possessing a Varying elastic force, the said constant elastIe torce being substantially four times the said varying elastic force of the said microphone.

6. A microphone of thetype described including a vibrating structure possessing a substantially constant elastic force and granular carbon filling means compressed by the'acti'on of the vibratory structure, the compressional force furnished by the granular carbon filling means being small compared to the elastic force of said vibratory structure.

7. A' microphone of the type described resistance varying means exerting a variable "pressure against said diaphragm, the effective mass of said resistance'varying means on said diaphragm being small as compared to the mass of said diaphragm.

8. A microphone of the type described having a diaphragm with a large central mass, resistance varying means exerting a variable pressure against said diaphragm, the effective mass of said resstance varying means on said diaphragm being small as compared to the mass of said diaphragm.

9. A microphone of the type described having a casing of relatively large mass, a

diaphragm mounted on said casing having a relatively large central mass and a resistance varyingmeans exerting a variable pressure over only a small area of said diaphragm, the effective mass of said resistance varying means on said diaphragm being small as compared to the mass of said diaphragm.

In testimony whereof I aflix my signature.

HUGO LICHTE. 

